This invention relates generally to organic Rankine cycle plants, and more particularly to methods and systems for maximizing power output or efficiency of waste heat recovery plants that employ organic Rankine cycles using variable speed generators and/or pumps and/or fans.
Rankine cycles use a working fluid in a closed cycle to gather heat from a heating source or a hot reservoir by generating a hot gaseous stream that expands through a turbine to generate power. The expanded stream is condensed in a condenser by rejecting the heat to a cold reservoir. The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The efficiency of Rankine cycles such as organic Rankine cycles (ORC)s in a low-temperature heat recovery application is very sensitive to the temperatures of the hot and cold reservoirs between which they operate. In many cases, these temperatures change significantly during the lifetime of the plant. Geothermal plants, for example, may be designed for a particular temperature of geothermal heating fluid from the earth, but lose efficiency as the ground fluid cools over time. Air-cooled ORC plants that use an exhaust at a constant temperature from a larger plant as their heating fluid will still deviate from their design operating condition as the outside air temperature changes with the seasons or even between morning and evening.
Waste heat recovery plants based on organic Rankine cycles are often required to work in harmony with different types of heat sources such as engines or turbines of different sizes and power levels. It would be advantageous to provide a control system and method for ensuring optimized organic Rankine cycle plant operation during mismatching temperature levels of the heat source(s) and for changing/mismatching heat load coming from the heat source(s) as well as for changing ambient conditions and fluid properties for waste heat recovery plants that employ variable speed generators and/or pumps and/or fans in which the waste heat recovery plant is based on organic Rankine cycles.